1. Field of the Invention
The present invention relates to emission monitoring and control of gases in various environments, and more specifically, it relates devices for in-situ gas detection.
2. Description of Related Art
Emission monitoring and control of gases in various environments is critical in determining the overall health of a given system. In particular, low-weight molecule sensing (e.g., H2, O2, CO2, NO2, C2H2) will indicate possible inward/outward leaks (Ar, O2, H2O, He), identify degradation (H2O→corrosion, NOx←He), identify aging mechanisms and incompatibilities, detect contamination and reactions (H2, NOx) and detect outgassing (COx, O2). Such system designs call for in-situ, minimally invasive, trace-gas analysis, with the requirement of parts per million (ppm) sensitivity, at a few atmospheres pressure of N2 air and within a cubic centimeter volume. In general, such sensors can benefit with on-board, smart self-calibrating subsystems to augment the basic device.
Examples of systems that can benefit from trace-gas sensors include compact and highly multiplexed autonomous systems for laboratory or field monitoring of nuclear, chemical or biological threats, for combustion and environmental research, pollution monitoring, space exploration, aircraft cabin atmosphere monitoring, nuclear proliferation detection, weapons aging, etc.
Presently, there exist a variety of sensors based on optical techniques, including optoelectronics and photonics, optical fiber sensors, as well as electrical and electromechanical technologies. Tables 1A and 1B tabulate prior art sensor techniques and properties as a function of selected device parameters. Tables 1C and 1D provide similar device characteristics, as applied to embodiments of the present invention. For each given sensor listed, entries in the table include its respective principles of operation, typical species to be sensed, performance, sensitivity, lifetime and other relevant device parameters, as well as representative archival references.
Referring to tables 1A and 1B, to further place the present invention within the context of state-of-the art sensors, are summarized relevant performance figures-of-merit in this survey of existing and promising technologies that have the potential to address typical applications issues, including the following: (1) low cross-sensitivity in the presence of other trace gases; (2) time-dependent concentration tracking; (3) limitations in terms of null outgassing; (4) chemical compatibility with nuclear components; (5) relative compactness; (6) degree of maintenance; (7) moving parts, if required; (8) device weight; (9) power operation and (10) reliability and lifetime.
The prior art entries are partitioned in tables 1A and 1B, based on the underlying technology and principles of operation, including the following: (1) Specialty Optical Fibers; (2) Advanced CMOS ICs and (3) optoelectronic and photonic devices. The first set of entries includes examples from the more mature and established class of sensors, such as NIR, mid-IR, and IR absorption techniques, with detection sensitivity in the range of 10 s of ppm, with very fast response times. Some examples include NO2 and CO, C2H2 detection using hollow fibers (see ref. i) and microbore fibers (see ref. ii), respectively. IR absorption studies of CO2 have been pursued using Fourier Transform IR techniques, FTIR (see ref. iii), and NO using Quantum Cascade (QC) lasers as optical probe sources (see ref. iv). The main issues with these systems include the device dimensions, usually in excess of a cubic centimeter, and fiber occlusion, which leads to poor reliability.
Within the second class of basic trace-gas technologies, electrical and electrochemical transducing techniques are included. In this class of sensor, the measured parameter, in the presence of the desired trace gas, is typically a change in resistance, capacitance or current. Fully integrated systems with temperature and relative humidity controls are currently produced by Keibali Corp. and Sandia/H2SCAN for H2 detection using Pd coated MOS capacitors, HEMT, or Schottky diodes with sensitivity down to 0.2% and milliseconds to seconds response times, with a few years of lifetime claimed (see ref. v). Other porous films of metal-oxides or metal-semiconductors with interdigitated electrode layouts have been used to demonstrate NO2, and COx detection below the ppm level (see ref. vi). The major application concern for this class of sensor is claimed to be the partial cross-sensitivity in the presence of other gas species, which can be reduced using high temperatures (thus the integration of micro-hotplates) and noble metals. Also, the response time of few minutes, dependent on the adsorption rates, is higher relative to competing techniques such as galvanic/electrochemical cell based sensors. Test cells, that are commercially available at BW Technologies and RKI Instruments, indicate the presence of a desired trace gas via changes in the measured current, as a result of ox-redox chemical reactions, with long lifetimes and wide dynamic ranges of detection. Their versatility to detect several gases, e.g., Ox, COx, and NOx, becomes an issue since the level of cross-sensitivity to different gases is very high. Porous Si FETs (see ref. vii) and thin films (see ref. viii) which, respectively, reveal a change in current and photoluminescence (PL), have a limitation in terms of saturation, or quenching. These sensors have been used for NOx sensing, with sensitivities in the range of <1 ppm levels but the latter are limited by PL recovery time and a chronic PL quench.
Finally, in the third category tabulated in tables 1A and 1B, are listed new nanotechnology approaches such as SERS (Surface Enhanced Raman Spectroscopy), an example of which exploits substrates with silver (Ag) nanoclusters (˜100 μm2). These sensors have been currently used for complex molecules detection, e.g., H2NO3 (see ref. ix), HE, CBW agents (see ref. x) and are presently also being considered for PH and simpler molecular detection (see ref. xi). The limit of the SERS approach appears to be film oxidation, which limits the lifetime and the complexity of readout systems. At present, the level of sensitivity remains relatively high (100 ppm). Hydrogen sensing has also been demonstrated by Zhao et al. (see ref. xii) using white-light reflectance on palladium-gold (PdAu) thin films and by Villatoro/Olpiski et al. (see ref. xiii) using Pd nanotapered fibers at 1.5 μm signal wavelength. These novel techniques seem to be promising, having demonstrated acceptable values in sensitivity, response time, and lifetime in their initial proof-of-concept demonstrations. In addition to the above approaches, there exists relatively mature optical-based (LED) systems for O2 sensing, manufactured at Ocean Optics (see ref. xiv). The underlying principle in this case is based on the quenching of oxygen-sensitive fluorescent dyes, such as Ruthenium (Rt) or porphyrin, embedded in thin films. A step forward is now offered by OLEDs, built at ISTI/Ames Labs and also developed at Tokai University, which are extremely appealing, given that the light source/sensor/controls/detector are integrated into a single miniaturized chip. In these last two examples, the devices are aimed at only one particular molecule. The device applicability and performance limitations of these sensors, which are, at present, still in the development phase, are principally functions of the selection and interaction dynamics of the given surface coatings with the trace-gas(es) of interest.
TABLE 1A(Prior art)MeasuredGasDetectionResponseCross-ConfigurationParameterSpecieRangeTimessensitivity0.25 m hollowIR abs/NIRNO2/CO,10-200 ppm0.02-7 syesfiber/microboreabsC2H2FTIR, 20 cm wdgIR absCO210-200 ppmyes9 m fiber QCIR absFinaNO 0.06 ppmyeslaserPd MOSΔC/CH20.2-100%75 ms-20 syescapacitors &ΔR/RHEMT/SchottkydiodesGalvanic/ElectroΔl/l by ox-Ox, COxN 0-999 ppm5-30 snochemical cellsred oxOx ...(cm3)Porous SiΔl/l/PLNOx  0.1-2 ppm10 s m (PLyesFET/Thin filmrecovery)Porous metal-ΔR/RNO2, COx 0.05-3 ppm1-15 mPartialoxides, metal-(adsorp.(high T,semic. IDTrates)nobleelectrodemetals)LED or OLEDDye fluorescenceO2 0-40 ppm1 μs-1 syesw/Dye-Thinquenchingfilm (100 μm2)fiber to spectrumHe—Ne, substr.WavelengthH2NO3  ~100 ppm30-60 myesw/Ag nanoRaman shiftsarin, HEclusters (100 μm2),ΔλRamanSpectrographWhite LightΔR/R/H20.2-4%/5-130 s/~10 syesPdAu thin filmΔT/T~10 sreflectance/Pdnanotaperedfiber (μm2 ×mm)λ = 1.5 μm
TABLE 1B(Prior art)OperationalConfigurationLifetimePointReferencesShortfalls0.25 m hollowXStandardSaito 1992Dimensionsfiber/microbore(fouling)Prickell 2004OcclusionDiffusionFTIR, 20 cm wdgXStandardKozodoy 1996Dimensions(fouling)OcclusionDiffusion9 m fiber QCXStandardFetzer 2003Dimensionslaser(fouling)OcclusionDiffusionPd MOSFewT&RHKeibali CorpCross-sensitivitycapacitors &yearscontrolH2scan-SNLAIrreversibilityHEMT/Schottky(MEMS)Time ResponsediodesLifetimeGalvanic/ElectroFewT&RHBW Techn/RKICross-sensitivitychemical cellsmos-few(MEMS)InstrumIrreversibility(cm3)yearsTime ResponseLifetimePorous SiLimitedStandardSberveflieri 2006Cross-sensitivityFET/Thin film(PL redSailor 1996Irreversibilityox)Time ResponseLifetimePorous metal-DependT control:Barrettino 2006,Cross-sensitivityoxides, metal-onuhotplateKim 2005Irreversibilitysemic. IDTsystem(cm3)Wang 2003,Time Responseelectrodefuturlec.comLifetimeLED or OLEDFew yrsStd/T&HROceanContendersw/Dye-ThincontrolOptics/ISTI_AmesEquipmentfilm (100 μm2)(MEMS)Labs/Tokai Un.Lifetimefiber to spectrumCoating developmentHe—Ne, substr.X (oxid)StandardStokes 2000, 2005Contendersw/Ag nanoTalley 2004Equipmentclusters (100 μm2),Yan 2005LifetimeRamanCoating developmentSpectrographWhite Light>1 yrStandard +Zhao 2005ContendersPdAu thin filmT&RHVillatoro 2005Equipmentreflectance/PdcontrolOlpiski 2004LifetimenanotaperedCoating developmentfiber (μm2 ×mm)λ = 1.5 μm